U.S. patent number 11,340,248 [Application Number 16/511,433] was granted by the patent office on 2022-05-24 for sensor packages.
This patent grant is currently assigned to ATLANTIC INERTIAL SYSTEMS LIMITED. The grantee listed for this patent is Atlantic Inertial Systems Limited. Invention is credited to Kiran Harish, Alan Malvern.
United States Patent |
11,340,248 |
Malvern , et al. |
May 24, 2022 |
Sensor packages
Abstract
A sensor package comprising: a sensor, wherein the sensor
comprises a sensing structure formed in a material layer and one or
more further material layers arranged to seal the sensing structure
to form a hermetically sealed sensor unit; a support structure; one
or more springs flexibly fixing the hermetically sealed sensor unit
to the support structure; wherein the one or more springs are
formed in the same material layer as the sensing structure of the
sensor unit; and one or more external package wall(s) encapsulating
the sensor unit, the support structure, and the one or more
springs, wherein the support structure is fixed to at least one of
the package wall(s). The springs decouple mechanical stresses
between the sensor unit and the external package wall(s) so as to
reduce the long term drift of scale factor and bias.
Inventors: |
Malvern; Alan (Plymouth,
GB), Harish; Kiran (Plymouth, GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
Atlantic Inertial Systems Limited |
Plymouth |
N/A |
GB |
|
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Assignee: |
ATLANTIC INERTIAL SYSTEMS
LIMITED (Plymouth, GB)
|
Family
ID: |
1000006325053 |
Appl.
No.: |
16/511,433 |
Filed: |
July 15, 2019 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20200025786 A1 |
Jan 23, 2020 |
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Foreign Application Priority Data
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Jul 20, 2018 [GB] |
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1811925 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01P
1/003 (20130101); G01P 1/023 (20130101) |
Current International
Class: |
G01P
1/02 (20060101); G01P 1/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2005070403 |
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Jul 2005 |
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KR |
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0055638 |
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Sep 2000 |
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WO |
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Other References
Extended European Search Report for International Application No.
19187276.1 dated Nov. 7, 2019, 9 pages. cited by applicant .
Intellectual Property Office Search and Examination Report for
International Application No. 1811925.5 dated Jan. 18, 2019, 6
pages. cited by applicant.
|
Primary Examiner: Lindsay, Jr.; Walter L
Assistant Examiner: Do; Andrew V
Attorney, Agent or Firm: Cantor Colburn LLP
Claims
The invention claimed is:
1. A sensor package comprising: a sensor, wherein the sensor
comprises a sensing structure formed in a material layer and one or
more further material layers arranged to seal the sensing structure
to form a hermetically sealed sensor unit; a substrate; a support
structure; one or more springs flexibly fixing the hermetically
sealed sensor unit to the support structure; wherein the one or
more springs are formed in the same material layer as the sensing
structure of the hermetically sealed sensor unit; one or more
external package wall(s) encapsulating the hermetically sealed
sensor unit, the support structure, and the one or more springs;
wherein the support structure is fixed to the substrate; and a
squeeze film damping structure arranged between the hermetically
sealed inertial sensor unit and the support structure.
2. The sensor package of claim 1, wherein the one or more springs
have a serpentine form.
3. The sensor package of claim 1, wherein the support structure is
formed in the same material layer as the sensing structure of the
hermetically sealed sensor unit and the one or more springs.
4. The sensor package of claim 1, wherein the support structure is
a frame surrounding the hermetically sealed sensor unit.
5. The sensor package of claim 1, wherein the support structure is
fixed to the substrate via a rigid mount.
6. The sensor package of claim 1, wherein the squeeze film damping
structure comprises a plurality of interdigitated damping
fingers.
7. The sensor package of claim 6, wherein the squeeze film damping
structure is formed in the same material layer as the sensing
structure of the hermetically sealed sensor unit and the
spring(s).
8. The sensor package of claim 1, further comprising flexible wire
bonds electrically connecting the hermetically sealed sensor unit
to at least one of the external package wall(s).
9. The sensor package of claim 1, wherein the hermetically sealed
sensor unit is electrically connected to at least one of the
external package wall(s) by an electrically conductive path carried
by the one or more springs.
Description
FOREIGN PRIORITY
This application claims priority to Great Britain Patent
Application No. 1811925.5 filed Jul. 20, 2018, the entire contents
of which is incorporated herein by reference.
TECHNICAL FIELD
The present disclosure relates to sensor packages, in particular to
MEMS sensor packages.
BACKGROUND
Sensors, for example pressure sensors or inertial sensors (such as
accelerometers and gyroscopes) are used in many applications,
including inertial navigation, robotics, avionics, and automobiles.
In inertial navigation applications, such sensors may be found in
self-contained systems known as "inertial measurement units"
(IMUs). IMUs typically contain a plurality of accelerometers and/or
gyroscopes, and provide an estimate of an object's travel
parameters such as angular rate, acceleration, altitude, position,
and velocity, based on the outputs of gyroscope(s) and/or
accelerometer(s). Each inertial sensor in an IMU is a
self-contained package. An IMU typically consists of accelerometers
and gyroscopes sensing in all three axes. This is normally part of
an Inertial Navigation System (INS), which adds computation of
velocity and position using navigation algorithms. At IMU level,
the outputs are usually limited to angular rotation and velocity
increments with each sample.
Microelectromechanical systems (MEMS)-based sensors, typically
fabricated from a single silicon wafer, can be used e.g. to measure
pressure or temperature, or linear or angular motion without a
fixed point of reference. MEMS pressure sensors often work on the
principle of mechanical deformation of a MEMS structure due to
fluid pressure. MEMS gyroscopes, or strictly speaking MEMS angular
rate sensors, can measure angular rate by observing the response of
a vibrating MEMS structure to Coriolis force. MEMS accelerometers
can measure linear acceleration by observing the response of a
proof mass suspended on a spring in a MEMS structure. High
performance MEMS inertial sensors are defined by their bias and
scale factor stability.
A MEMS sensor is usually supported on an isolation layer within its
package. For example, an isolation layer of silicone elastomer may
be provided between the package and the lowermost glass layer of
the MEMS sensor. In some examples, the MEMS sensor may be mounted
on an isolation layer including a raft that is connected to the
surrounding package via springs or other damping structures. The
isolation layer has two main functions: to provide isolation from
unwanted external vibrations; and to absorb mechanical stress due
to thermal expansion differences between the MEMS sensor and the
surrounding package (typically alumina or ceramic).
The stability of the isolation layer that attaches a MEMS inertial
sensor to its package is important for high performance, especially
when trying to achieve better than 0.1 mg bias stability. An
elastomeric isolation layer is usually chosen to have a very low
elastic modulus (e.g. silicone) to decouple the MEMS sensor from
package stresses. However, such materials suffer from long term
creep and ageing effects which can therefore alter sensor
performance (e.g. bias and scale factor) by virtue of stress relief
over a period of time in service. It is therefore difficult to
achieve good isolation of an inertial sensor from package stresses
and good long term stability in performance. Similar considerations
apply when mounting any MEMS sensor in a package.
There remains a need for improved isolation mounting in sensor
packages.
SUMMARY
According to a first aspect of this disclosure, there is provided a
sensor package comprising: a sensor, wherein the sensor comprises a
sensing structure formed in a material layer and one or more
further material layers arranged to seal the sensing structure to
form a hermetically sealed sensor unit; a support structure; one or
more springs flexibly fixing the hermetically sealed sensor unit to
the support structure; wherein the one or more springs are formed
in the same material layer as the sensing structure of the sensor
unit; and one or more external package wall(s) encapsulating the
sensor unit, the support structure, and the one or more springs,
wherein the support structure is fixed to at least one of the
package wall(s).
According to a second aspect of this disclosure, there is provided
a method of manufacturing a sensor package, the method comprising:
forming a sensing structure in a material layer; forming one or
more springs in the same material layer as the sensing structure;
adding one or more further material layers to seal the sensing
structure to form a hermetically sealed sensor unit with the one or
more springs flexibly fixing the hermetically sealed sensor unit to
a support structure; and fixing the support structure to one or
more external package wall(s), the one or more external package
wall(s) encapsulating the inertial sensor unit, the support
structure, and the one or more springs.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a prior art sensor package that uses a full
elastomeric die bond.
FIG. 2 shows a sensor package in accordance with an example of the
present disclosure.
FIG. 3 shows a plan view of a first material layer in accordance
with an example of the present disclosure.
FIG. 4 shows a further plan view of the first material layer,
including squeeze damping fingers, in accordance with an example of
the present disclosure.
FIGS. 5a-5k show a process for manufacturing a sensor package in
accordance with an example of the present disclosure.
FIGS. 6a-6k show a process for manufacturing a sensor package in
accordance with another example of the present disclosure.
DETAILED DESCRIPTION
The present disclosure relates to a sensor package and a method of
manufacturing a sensor package. It will be appreciated that forming
one or more springs in the same material layer as the sensing
structure of the sensor unit is a completely different approach to
sensor isolation than the conventional way of mounting the
hermetically sealed sensor unit to an external package using a
different material, e.g. an elastomeric material, as an isolation
layer. The springs can decouple mechanical stresses between the
sensor unit and the external package wall(s) so as to reduce the
long term drift of scale factor and bias. Furthermore, the effects
of temperature sensitivity on bias and scale factor can also be
reduced e.g. for an inertial sensor.
Some non-limiting examples of a sensor package and a method of
manufacturing a sensor package are described in further detail
below.
FIG. 1 shows a prior art inertial sensor package 100 comprising an
upper glass layer 102, a silicon sensing layer 104, a lower glass
layer 106, an elastomeric die bond layer 110, an alumina substrate
112, and a package lid 114. The three layers 102, 104 and 106 make
up a hermetically sealed inertial sensor unit 108.
The silicon sensing structure in the layer 104 is sensitive to some
form of applied force, in this case, acceleration. This silicon
sensing layer 104 is sandwiched between the upper glass layer 102
and the lower glass layer 106 to form a hermetically sealed
inertial sensor unit 108. This sealing allows for the silicon
sensing structure in the layer 104 to occupy a space within a
controlled environment. The inside of the hermetically sealed
inertial sensor unit 108 comprises an atmosphere that is controlled
to optimise sensor performance.
The hermetically sealed inertial sensor unit 108 is attached to the
alumina substrate 112 by elastomeric die bond layer 110. As shown,
the elastomeric die bond layer 110 covers all of the lower glass
layer 106 of the hermetically sealed inertial sensor unit 108. The
elastomeric die bond layer 110 is flexible. This provides good
adhesion for the hermetically sealed inertial sensor unit 108 to
the alumina substrate 112, and further provides good mechanical
stability under shock and vibration of the inertial sensor package
100.
However, as described in the background section above, thermal and
mechanical stresses across the inertial sensor package 100 can
adversely affect the performance of inertial sensor unit 108,
especially later in the lifetime of the device.
Package lid 114 allows the hermetically sealed inertial sensor unit
108 to be further protected from external environmental influences,
such as dirt or direct force application. Typically, this lid is a
metal alloy or alumina and is soldered to the substrate 112. The
package formed by lid 114 also forms a hermetic seal, and allows
for controlling the gaseous environment inside the package. This is
done to ensure an optimal dry gas environment inside the package,
and prevents any moisture ingress, which may negatively affect
sensor performance, particularly in a capacitive-type inertial
sensor.
FIG. 2 shows a sensor package, e.g. an inertial sensor package 200
in accordance with an example of the present disclosure. The
boundaries of the inertial sensor package 200 are defined by three
external walls 216, and a substrate 220. The inertial sensor
package 200 comprises a silicon sensing structure 204 formed in a
silicon material layer 202 for detecting an applied acceleration.
The silicon sensing structure 204 is hermetically sealed into an
inertial sensor unit 210, by an upper glass layer 206 and a lower
glass layer 208. The inertial sensor package 200 also comprises a
support structure 214 formed in the silicon layer 202. The support
structure 214 takes the form of a frame surrounding the
hermetically sealed inertial sensor unit 210. The support structure
214 is attached to the substrate 220, through the intervening glass
layer 208, by a compliant or fixed mount 218. The inertial sensor
unit 210 is decoupled from the substrate 220, as it is instead
suspended from the support structure 214 by a plurality of springs
212, formed in the silicon material layer 202. In this example, the
silicon sensing structure 204 is electrically connected to an
external connection 226 via flexible wire bonds 224, and
through-hole vias 222 in the upper glass layer 206.
As shown, the sensor unit 210 is decoupled from stresses and sudden
forces applied to the package 200 in two ways. Primarily, the
springs 212 that join the `floating` sensor unit 210 to the support
structure 214 compensate for any such stresses, leaving the sensor
unit 210 free from such imbalances, decreasing long term drift of
scale factor and bias. Furthermore, the effects of temperature
sensitivity on bias and scale factor can also be reduced. Secondary
to the effects of the springs 212, the elastomeric mount 218 can
also absorb some stresses and shocks to the package 200. In these
ways, sensor performance is improved.
As shown, parts of the support structures 214, the springs 212, and
the sensing structure 204 are all made in the same silicon material
layer 202. This may have significant benefits to the manufacturing
process, as streamlined development can take place, enabling the
device to be mostly manufactured before needing to singulate the
parts from a wafer. This batch processing of the devices may both
increase throughput and decrease cost of manufacture. By
manufacturing the devices this way, the sensor unit 210 can be
conveniently decoupled from the support structures 214 by etching
out the springs 212 during the same process that is used to etch
out the sensing structure 204.
An electrical connection is made to the sensing structure 204 with
a conductive e.g. metal path passing down the through-hole vias
222. This connection is then carried down to the substrate 220, via
the flexible wire bonds 224, to meet the external connections 226
which allow an electrical connection to be made from the outside of
the sensor package 200 directly to the sensing structure 204. The
flexible wire bonds 224 are flexible enough to withstand any
stresses or gradients within the package, for example flexing of
the springs 212.
The upper 206 and lower 208 glass layers form a hermetic seal
around the sensor unit 210. The environment within the sensor unit
210 can be controlled at the time of sealing, and in this example,
the sensor is filled with dry Nitrogen at atmospheric pressure.
This controlled environment enables tuning of the damping factor
within the hermetically sealed sensor unit 210.
The external walls 216 form a hermetic seal around the support
structures 214 and the sensor unit 210. The environment within this
area can be controlled upon sealing as well. The control of this
environment enables tuning of the damping factor of the squeeze
film damping fingers (not shown in FIG. 2), which is explained in
more detail below with reference to FIG. 4.
FIG. 3 shows a plan view of the first material layer in accordance
with an example of the present disclosure. The first material layer
is a silicon layer 300. Silicon layer 300 is made from a single
sheet of crystalline silicon. The silicon layer 300 comprises a
support structure 302, taking the shape of an outer frame, a
plurality of springs 304, and a sensing structure 306. There is no
residual glass in the spring area. The support structure 302 is a
frame surrounding the sensing structure 306.
As shown, the sensing structure 306 is suspended from the support
frame 302 by the springs 304. The springs 304 decouple the sensing
structure 306 from the mechanical and other stresses experienced by
the inertial sensor package 200, whilst having a spring constant
such that inertial movement is still transferred to the sensing
structure 306. The resonant frequency of the springs 304 is between
1-5 kHz, for example 2 kHz. The length and width of the springs 304
is designed in order to select an optimal resonant frequency. The
springs 304 are serpentine in shape, and have a number of
serpentine turns, for example between 1-10 turns.
The manufacture of the support structure 302, springs 304 and
sensing structure 306 all in a single material layer allows for a
streamlined manufacturing process, saving both cost and time.
FIG. 4 shows a further plan view of the first material layer 400,
including optional squeeze film damping fingers 406, in accordance
with an example of the present disclosure. Shown in FIG. 4 is a
support structure 402, a sensing structure 408, springs 404, and
squeeze film damping fingers 406.
As in FIG. 3, the springs 404 suspend the sensing structure 408
from the support structure 402. The addition of the squeeze film
damping fingers 406 assists in the decoupling of mechanical and
other stresses between the inertial sensor package and support
structure, and the sensing structure 408. The squeeze film damping
fingers 406 help to provide near critical damping to the inertial
sensor package, preventing the sensing structure 408 from being
damaged. The squeeze film damping fingers 406 do this by limiting
the range of movement of the sensing structure 408 with respect to
the support structure 402.
The damping effect of the squeeze film damping fingers 406 can be
tuned by altering the composition of the inertial sensor package
environment, for example by filling it with dry Nitrogen, Neon or
Argon at atmospheric pressure. The damping effect can also be tuned
by adjusting the number of fingers, the lengths of the fingers, and
the size of the gaps between the fingers.
FIGS. 5a-5k shows a process for manufacturing a sensor package in
accordance with an example of the present disclosure.
FIG. 5a shows the first step of pre-cavitating a layer of glass 502
with a wet etch. The etch is defined by a mask, and the glass layer
502 is only etched in the region which a moving sensing structure
will later occupy. The depth of the etch is typically around 30
.mu.m.
FIG. 5b shows the next step of anodically bonding a silicon wafer
504 to the glass layer 502.
FIG. 5c shows the next step of creating through-hole vias 506. This
is typically done by first applying a photomask (not shown), and
powder blasting the glass layer 502 in order to create the
through-hole vias 506. The through-hole vias 506 go through to the
silicon layer 504, inside the pre-cavitated area of the glass layer
502.
FIG. 5d shows the next step of depositing a metal tracking layer
508 onto the glass layer 502, forming a uniform thin layer 508
coating the glass layer 502, and the inside surfaces of the
through-hole vias 506 in the process. Alternatively, the metal
tracking layer 508 may fill the through-hole vias 506. This allows
electrical connections to be made to the silicon layer 504, in
order to connect a sensing structure made in the silicon layer 504
with an external package. The metal tracking layer 508 is typically
deposited and then patterned by photo-lithography.
FIG. 5e shows the next step of performing an isotropic wet etch on
the glass layer 502, in the regions 510 where the springs will be
formed, in order to suspend the sensing structure later formed in
the silicon layer 504. A photomask (not shown) is used to protect
the other areas from the wet etch. This exposes the underlying
silicon layer 504.
FIG. 5f shows the next step of performing a Deep Reactive Ion Etch
(DRIE) on the underlying silicon layer 504 from the bottom. A
standard photo mask (not shown) is used to define the etched
regions of the silicon layer 504. In this step, a sensing structure
512 is etched from the silicon layer 504. This etch also defines a
plurality of serpentine springs 514 suspending the sensing
structure 512 from the newly defined support structure 516.
Next, a lower glass layer 518 is pre-cavitated in moving regions of
the sensing structure 512 in the silicon layer 504. As shown in
FIG. 5g, the lower glass layer 518 is then anodically bonded to the
silicon layer 504, forming a hermetically sealed sensor unit 520
containing the sensing structure 512. The hermetically sealed
sensor unit 520 is back-filled with a gas, typically dry Nitrogen,
Argon or Neon at atmospheric pressure. This ensures near critical
damping of the sensing structure 512.
FIG. 5h shows the next step of performing an isotropic wet etch on
the lower glass layer 518 to the depth of the silicon layer 504.
This etch is defined by a photomask (not shown), and leaves only
the springs 514 in the regions 510 of FIG. 5(e). This also releases
the hermetically sealed sensor unit 520 from the support structure
516, leaving it suspended by the springs 514. This decouples the
hermetically sealed sensor unit 520 from any large shocks or
stresses experienced by the support structure 516, as they will be
absorbed by the springs 514 instead. Furthermore, it will be seen
that the sensing structure 512 is hermetically isolated from the
springs 514 by the anodically bonded glass layers 502, 518, forming
the hermetically sealed sensor unit 520. The device may be
singulated from the wafer after this stage too. This allows for
streamlining of production of the devices, as the devices are
almost fully formed before they are singulated from the wafer.
FIG. 5i shows the next step of bonding the support structures 516
to a substrate 524 (typically made from alumina or ceramic), via
one or more elastomeric mounts 522 (for instance). In this way, the
hermetically sealed sensor unit 520 is decoupled from any stresses
or shocks experienced by the substrate 524 via the springs 514 as
well as the elastomeric mounts 522.
FIG. 5j shows the next step of adding flexible wire bonds 526 to
the device, thereby attaching the metal tracking layer 508 on the
hermetically sealed sensor unit 520 to the (relatively) fixed
support structures 516. Furthermore, flexible wire bonds 526 are
also added from the support structure 516 to the substrate 524. The
wire bonds typically have a diameter of 25 .mu.m. An external
electrical connection 525 is also added through the substrate layer
524.
FIG. 5k shows the final step of adding a metal lid 528,
hermetically sealing the internal gas volume of the package. This
is typically done using solder sealing (at .about.300.degree. C.),
securing the lid 528 to the substrate 524. The internal gas volume
of the package is controlled to optimise the decoupling between the
hermetically sealed sensor unit 520 and the support structures 516,
and typically comprises Argon, Neon or dry Nitrogen at atmospheric
pressure--e.g. to optimise squeeze film damping.
FIGS. 6a-6k show a process for manufacturing a sensor package in
accordance with another example of the present disclosure. The
manufacturing process shown in FIGS. 6a-6ak is similar to that
shown in FIGS. 5a-5k, and will be described below with reference to
FIGS. 5a-5k where appropriate.
The manufacturing process shown in FIGS. 6a-6c is the same as that
shown in FIGS. 5a-5c.
FIG. 6d shows the next step of performing an isotropic wet etch on
the glass layer 602, in the regions 610 where the springs will be
formed, in order to suspend the sensing structure. A photomask (not
shown) is used to protect the other areas from the wet etch. This
exposes the underlying silicon layer 604.
FIG. 6e shows the next step of performing a DRIE on the underlying
silicon layer 604 from the bottom. A standard photo mask (not
shown) is used to define the etched regions of the silicon layer
604. In this step, a sensing structure 612 is etched from the
silicon layer 604. This etch also defines the serpentine springs
614 suspending the sensing structure 612 from newly defined support
structure 616.
FIG. 6f shows the next step of depositing a metal tracking layer
608 onto the silicon layer 604 and the glass layer 602, forming a
uniform thin layer 608 coating the glass layer 602, and the inside
surfaces of the through-hole vias 606 in the process.
Alternatively, the metal tracking layer 608 may fill the
through-hole vias 606. This allows electrical connections to be
made to the silicon layer 604, in order to connect a sensing
structure made in the silicon layer 604 with an external package.
This step also defines metal tracking down the faces of the
isotropically etched glass layer 602, and across the surface of the
springs 614. This provides an electrically conductive path from the
sensing structure 612 and the through-hole vias 606, to the edge of
the glass layer 602 where the support structure 616 surrounds the
sensing structure 612. The metal tracking layer 608 is typically
deposited, and then patterned by photo-lithography.
Next, a lower glass layer 618 is pre-cavitated in moving regions of
the sensing structure 612 in the silicon layer 604. As shown in
FIG. 6g, the lower glass layer 618 is then anodically bonded to the
silicon layer 604, forming a hermetically sealed sensor unit 620
containing the sensing structure 612. The hermetically sealed
sensor unit 620 is back-filled with a gas, typically dry Nitrogen,
Argon or Neon at atmospheric pressure. This ensures near critical
damping of the sensing structure 612.
FIG. 6h shows the next step of performing an isotropic wet etch on
the lower glass layer 618 to the depth of the silicon layer 604.
This etch is defined by a photomask (not shown), and leaves only
the springs 614 and the corresponding metal tracking in the regions
610 of FIG. 6(d). This also releases the hermetically sealed sensor
unit 620 from the support structure 616, leaving it suspended by
the springs 614. This decouples the hermetically sealed sensor unit
620 from any large shocks or stresses experienced by the support
structure 616, as they will be absorbed by the springs 614 instead.
Furthermore, it will be seen that the sensing structure 612 is
hermetically isolated from the springs 614 by the anodically bonded
glass layers 602, 618, forming the hermetically sealed sensor unit
620. The device may be singulated from the wafer after this stage
too. This allows for streamlining of production of the devices, as
the devices are almost fully formed before they are singulated from
the wafer.
FIG. 6i shows the next step of bonding the support structure 616 to
a substrate 624 (typically made from alumina or ceramic), via one
or more elastomeric mounts 622 (for instance). In this way, the
hermetically sealed sensor unit 620 is decoupled from any stresses
or shocks experienced by the substrate 624 via the springs 614 as
well as the elastomeric mounts 622.
FIG. 6j shows the next step of adding a flexible wire bond 626 to
the device, attaching the metal tracking layer 608 at the support
structure 616 to the substrate layer 624. The wire bonds typically
have a diameter of 25 .mu.m. An external electrical connection 625
is also added through the substrate layer 524. As previously
mentioned, this allows for an external electrical connection to be
made to the sensing structure 612, but in this example across the
electrically conductive paths carried by the springs 614 between
the flexible wire bond 626 and the sensor unit 620.
FIG. 6k shows the final step of adding a metal lid 628,
hermetically sealing the internal gas volume of the package. This
is typically done using solder sealing (at .about.300.degree. C.),
securing the lid 628 to the substrate 624. The internal gas volume
of the package is controlled to optimise the decoupling between the
hermetically sealed sensor unit 620 and the support structure 616,
and typically comprises Argon, Neon or dry Nitrogen at atmospheric
pressure--e.g. to optimise squeeze film damping.
It will be appreciated that forming one or more springs in the same
material layer as the sensing structure provides for ease of
manufacture while also decoupling mechanical and thermal stresses
between the sensor unit and the external package wall(s). More
generally, some examples of a sensor package and a method of
manufacturing a sensor package according to the present disclosure
are provided below.
According to one or more examples of the present disclosure, the
one or more springs may have a serpentine form. The geometrical
form of the springs may be designed to provide a predefined spring
compliance or stiffness. In at least some examples, the one or more
springs are configured to provide a spring resonance .gtoreq.1 kHz
and preferably in the range of 1-5 kHz. This has been found by the
inventors to give enough compliance without compromising sensor
performance at lower frequency.
According to one or more examples of the present disclosure, the
one or more springs preferably comprises a plurality of springs.
The springs may be arranged around the sensor unit. For example,
the sensor unit may be arranged centrally within the support
structure and the springs may extend in multiple directions between
the sensor unit and the support structure. The support structure
may be in the same plane as the sensor unit or in a different
plane, above and/or below the sensor unit. In one or more examples,
the sensor unit may be suspended by the springs fixing the sensor
unit to the support structure.
According to one or more examples of the present disclosure, the
material layer in which the sensing structure is formed comprises
silicon. The one or more springs may therefore be formed in the
same silicon layer as the sensing structure of the sensor unit. The
silicon springs can be shaped and/or dimensioned to give radial
compliance to allow for stress relief between the sensor unit and
the support structure. The silicon springs may conveniently be
etched out during the same process that is used to etch out the
sensing structure. For example, the one or more springs may be
formed by etching a serpentine form in the silicon material
layer.
According to one or more examples of the present disclosure, the
hermetically sealed sensor unit comprises a glass layer, a silicon
material layer comprising the sensing structure, and a further
glass layer. Such a material structure is known as a
silicon-on-glass (SOG) structure. The one or more further material
layers arranged to seal the sensing structure to form a
hermetically sealed sensor unit may therefore be glass
layer(s).
According to one or more examples of the present disclosure, the
hermetically sealed sensor unit comprises a silicon layer, a
silicon material layer comprising the sensing structure, and a
further silicon layer. The one or more further material layers
arranged to seal the sensing structure to form a hermetically
sealed sensor unit may therefore be silicon layer(s).
According to one or more examples of the present disclosure, the
support structure is formed in the same material layer as the
sensing structure of the sensor unit and the spring(s). In such
examples the support structure is in the same plane as the material
layer. This means that the support structure may be conveniently
decoupled from the sensing structure by etching out the spring(s)
during the same process that is used to etch out the sensing
structure. The support structure may therefore be formed from
silicon, the same as the sensing structure.
According to one or more examples of the present disclosure, the
support structure is a frame. The frame may surround the
hermetically sealed sensor unit. As mentioned above, a plurality of
the springs may extend between the sensor unit and the frame e.g.
suspending the sensor unit centrally within the frame.
According to one or more examples of the present disclosure, the
support structure is fixed to at least one external package wall
via a compliant (e.g. elastomeric) mount. Such an elastomeric mount
may provide a degree of compliance, but it will be appreciated that
the main decoupling between the sensor unit and the support
structure is through the one or more springs. The compliant mount
may require much less elastomeric material than the conventional
elastomeric isolation layer used in prior art sensor packages.
According to one or more alternative examples of the present
disclosure, the support structure is fixed to at least one external
package wall via a rigid mount. It will be appreciated that a rigid
mount may be used as the sensor unit is already decoupled from the
support structure through the one or more springs. The rigid mount
may comprise an adhesive e.g. epoxy bond or a metal solder
joint.
According to one or more examples of the present disclosure, the
sensor package further comprises a squeeze film damping structure
arranged between the hermetically sealed sensor unit and the
support structure. Such a damping structure comprises one or more
gaps that are sized so as to provide a squeeze film damping effect
in the gaseous atmosphere within the package, as is known in the
art. For example, the squeeze film damping structure may comprise a
plurality of interdigitated damping fingers. The plurality of
interdigitated damping fingers may be arranged in one or more sets,
for example multiple sets arranged around the sensor unit.
According to one or more examples of the present disclosure, the
squeeze film damping structure is formed in the same material layer
as the sensing structure of the sensor unit and the spring(s). This
means that the squeeze film damping structure may conveniently be
formed during the same process that is used to etch out the sensing
structure and the spring(s). The squeeze film damping structure
(e.g. interdigitated damping fingers) may therefore be formed from
silicon, the same as the sensing structure.
According to one or more examples of the present disclosure, the
hermetically sealed sensor unit is evacuated. According to one or
more alternative examples of the present disclosure, the
hermetically sealed sensor unit comprises a first gaseous
environment e.g. comprising one or more of Argon, Neon or dry
Nitrogen. The first gaseous environment may be at a pressure below
atmospheric pressure, e.g. partially evacuated. Alternatively, the
first gaseous environment may be at a pressure above atmospheric
pressure. This elevated pressure may give a higher damping
factor.
According to one or more examples of the present disclosure, the
sensor package comprises a second gaseous environment outside the
hermetically sealed sensor unit, e.g. made up of one or more of
Argon, Neon or dry Nitrogen. The second gaseous environment may be
at atmospheric pressure. In examples wherein a squeeze film damping
structure is arranged between the hermetically sealed sensor unit
and the support structure, the second gaseous environment may be
chosen to provide the desired squeeze film damping effect.
According to one or more examples of the present disclosure, the
sensor package further comprises flexible wire bonds electrically
connecting the sensor unit to at least one of the external package
wall(s). The hermetically sealed sensor unit may further comprise
one or more through-hole vias for electrical connection to the
sensing structure. This means that direct wire bonds may pass down
the through-hole vias to provide for electrical connection of the
sensing structure.
According to one or more examples of the present disclosure, the
hermetically sealed sensor unit is electrically connected to at
least one of the external package wall(s) by an electrically
conductive path carried by the one or more springs. For example,
conductive (e.g. metal) tracking may be carried by the one or more
springs. The hermetically sealed sensor unit may further comprise
one or more through-hole vias for electrical connection to the
sensing structure. This means that direct wire bonds may pass down
the through-hole vias to provide for electrical connection of the
sensing structure. This connection can then be linked to the
electrically conductive path carried by the one or more springs in
order to provide an electrical connection from at least one of the
external package walls to the sensing structure.
According to one or more examples of the present disclosure, the
sensor is a MEMS sensor.
According to one or more examples of the present disclosure, the
sensor is a pressure sensor. According to one or more other
examples of the present disclosure, the sensor is an inertial
sensor. It follows that the hermetically sealed sensor unit may be
a hermetically sealed inertial sensor unit.
According to one or more examples of the present disclosure, the
inertial sensor is a gyroscope. The sensing structure may comprise
a proof mass in the form of a disc or ring. The gyroscope may be a
vibrating structure gyroscope.
According to one or more examples of the present disclosure, the
inertial sensor is an accelerometer. The sensing structure may
comprise a fixed substrate and a proof mass mounted to the fixed
substrate by flexible support legs.
According to one or more further examples of the present
disclosure, the accelerometer is one of the following: a capacitive
accelerometer, an inductive accelerometer, or a piezoelectric
accelerometer. In at least some examples, the capacitive
accelerometer comprises: a fixed substrate and a proof mass mounted
to the fixed substrate by flexible support legs for in-plane
movement along a sensing axis in response to an applied
acceleration; the proof mass comprising a plurality of sets of
moveable electrode fingers extending substantially perpendicular to
the sensing axis and spaced apart along the sensing axis; at least
two pairs of fixed capacitive electrodes, wherein a first pair of
the fixed capacitive electrodes comprises a first fixed electrode
and a fourth fixed electrode, and a second pair of the fixed
capacitive electrodes comprises a second fixed electrode and a
third fixed electrode, and wherein each fixed capacitive electrode
comprises a set of fixed capacitive electrode fingers extending
substantially perpendicular to the sensing axis and spaced apart
along the sensing axis; wherein the sets of fingers of the first
and third fixed electrodes are arranged to interdigitate with the
sets of moveable electrode fingers with a first offset in one
direction along the sensing axis from a median line between
adjacent fixed fingers, and the sets of fingers of the second and
fourth fixed electrodes are arranged to interdigitate with the sets
of moveable electrode fingers with a second offset in the opposite
direction along the sensing axis from a median line between
adjacent fixed fingers.
In one or more examples, the method may further comprise: forming
the support structure in the same material layer as the sensing
structure of the inertial sensor unit and the one or more springs.
As is mentioned above, this is advantageous as the support
structure, spring(s) and sensing structure may all be formed from
the same material layer by a common manufacturing process such as
DRIE.
In one or more examples, the method may further comprise:
connecting flexible wire bonds between the sensor unit and at least
one of the external package wall(s).
In one or more examples, the method may further comprise: forming
an electrically conductive path across the one or more springs. The
electrically conductive path may be formed such that the
hermetically sealed sensor unit is electrically connected to at
least one of the external package wall(s). For example, the method
may further comprise: adding conductive (e.g. metal) tracking to a
surface of the one or more springs. The electrically conductive
path may be used to take a signal from the sensing structure to the
outer frame.
In one or more examples, the method may further comprise: forming a
squeeze film damping structure between the hermetically sealed
inertial sensor unit and the support structure. Preferably the
squeeze film damping structure is formed in the same material layer
as the sensing structure of the inertial sensor unit and the
spring(s).
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